On a late October day, the wind has blown most of the leaves off the trees. They crunch underfoot, reminding me of childhood. The ancient elms near our school dormitory used to provide vast quantities of leaves to play in. We would jump and roll in them, relishing that autumn crunch. I’m sure the grounds maintenance people gathered them up and burned them.
Today the fallen leaves lie on the yard. We don’t rake them up and we don’t burn them. We leave them to their natural end. But what exactly is that? Where do they go? What happens to them?
Well, there’s a whole series of things that happen to them, and there’s a whole cast of characters comprising hundreds of different species involved in their disappearance.
The first players in our cast of characters are fungi. They are responsible for reducing up to 75% of the mass of the leaves. They do this by producing enzymes that break down the cell walls in the leaf. Then the fungi absorb the leaf’s nutrients through the fungi’s thread-like network of hyphae. A leaf has several different types of cells that the fungi must address. Each species of fungi produces a specific enzyme that breaks down a specific cell type. For example, Specific types of fungi produce enzymes that break down cellulose while others address tannins, lignin, chitin, starch or pectin, all components of a leaf. The fungi arrive in a choreographed sequence to take their turns at the leaf. The first wave of fungi break down the easiest materials to get at the sugars, and the later waves tackle the tougher materials such as lignin (a component of wood). This process also makes those nutrients available to other organisms.
Bacteria come next. Some of the bacteria are there to eat what the fungi have unlocked. Some are there to eat the fungi, especially its dead parts. Some species of bacteria live in the guts of soil-dwellers who eat the leaves.
The third group of participants in the disappearance of our leaves is the critters that live in the leaf litter. These range from tiny nematodes and springtails to relatively giant spiders and earth worms. Also at the table are mites, insects, centipedes, insect larvae, etc. Some of them shred the leaf material. Some of them just eat it. And some of them clean up after the rest of them by eating their waste products.
The time it takes to break down this year’s leaves will depend on several factors. First of all, the leaves of different species decompose at different rates. For example, the leaves of deciduous trees may decompose within a year while pine needles can take up to seven years to decompose. If the weather is warm and wet but not too wet, the whole process may take a couple of years, although by spring most of the hard work will have been done. If the weather warms up, the process goes faster, and if it’s either too dry or too wet, the process slows down. Leaves that are chopped up also decompose faster than whole leaves.
Getting rid of the leaves in the fall is a major preoccupation with most lawn owners. The EPA estimates that 20% of all landfill waste is bagged-up dead leaves and garden plants. As they decompose in the landfill, the nutrients they contain such as nitrogen are not recycled in the lawn or garden.
Another time-honored method, of course, is to burn the leaves. The smell of burning leaves is iconic for fall, but burning the leaves doesn’t capture the nutrients for the soil either, and burning them sends their carbon right back into the atmosphere as carbon dioxide.
One way people speed up the decomposition process is by composting the leaves. Composting brings all the players together and adds heat to speed things up. Using the leaves as mulch in the garden or flower bed has several benefits. Mulch preserves the leaves’ nutrients for their native soil, maintains moisture in the soil, and keeps the weeds down in the garden.
All those dead leaves play continuing roles in the environment. Left in place, they create a protective shield against microhabitat fluctuations, soil erosion, and compaction. The leaf litter creates a microclimate favorable to fungi and other decomposers. Under the leaves, a variety of small creatures find shelter and food. Fallen leaves also provide nesting materials for birds and small animals, and holds rainwater, keeping the ground moist. In this protected environment, all the players working on recycling are able to thrive and do their jobs.
We don’t exactly have a garden just now, so we’ll let this year’s leaves go through their natural decomposition cycle. By spring when the grass starts to grow again, it can make use of some of those nutrients from last year’s leaves, and for all intents and purposes, the leaves will have disappeared in the yard.
Photo by Author. Alt text: View looking toward the east fencerow in late October. The red maple in the foreground has dropped its leaves, forming a litter for the cleanup crew to start their work.